1. Field of the Invention
The present invention relates to a DCxe2x80x94DC converter and, in particular, to a DCxe2x80x94DC converter, which utilizes a switching regulator inputted with and selectively driven by a voltage of a battery power source, a power source for obtaining DC power by rectifying AC power or an AC adapter and which has a wide dynamic range of an input voltage and can stabilize an output voltage thereof even when the power source voltage is varied. In the following description, the power source for generating DC power by rectifying AC power or the AC adapter will be referred to as xe2x80x9cAC power sourcexe2x80x9d.
2. Description of the Prior Art
In a conventional portable electronic device such as a portable audio equipment, a personal computer, a PHS (Personal Handyphone System), a portable telephone set or a PDA (Personal Digital Assistance), etc., it has been usual to utilize a DCxe2x80x94DC converter constructed with a switching regulator for obtaining a predetermined power source voltage by efficiently performing a power conversion.
FIG. 2 shows a construction of an example of a conventional switching regulator used for the DCxe2x80x94DC converter.
In FIG. 2, the switching regulator 10 includes an error amplifier (Err) 11, a reference voltage generator circuit 12, a PWM pulse generator circuit 13, which includes a comparator 13a and a triangular wave generator circuit 13b, a driver 14, a switching circuit 15 constructed with a P channel MOSFET Q and a Schottky diode D connected in series with the P channel MOSFET Q. The series circuit of the P channel MOSFET Q and the Schottky diode D is provided between a power source line +Vcc connected to an input side DC power source and ground GND.
The switching regulator 10 further includes an output capacitor C provided between an output terminal 16 and ground GND and a solenoid L is connected between a junction of the MOSFET Q and the Schottky diode D and the output terminal 16. In this case, a value of the solenoid L is in the order of 10 xcexcH and a capacitance of the output capacitor C is about 150 xcexcF, for example. The switching regulator 10 further includes a resistive voltage divider circuit 17 constructed with a series connection of resistors R1 and R2, which are provided between the output terminal 16 and ground GND. A detection voltage Vs detected by the resistive voltage divider circuit 17 is fedback to the error amplifier 11. The detection voltage Vs is compared by the error amplifier 11 with a reference voltage Vref generated by the reference voltage generator circuit 12 and a resultant voltage difference Ve (error detection signal) is inputted to the comparator 13a of the PWM pulse generator circuit 13.
The resistive voltage divider circuit 17 for generating the output voltage is constructed with the series circuit of the resistors R1 and R2 and a speed-up circuit for reducing a time period from an operation start time of the DCxe2x80x94DC converter 10 up to a time at which the DCxe2x80x94DC converter 10 becomes in a steady operation state. The speed-up circuit comprises a CR time constant circuit 17a, which is a series circuit of a resistor and a capacitor and is connected in parallel to the resistive voltage divider circuit 17 and functions to set a PWM driving gain.
The error amplifier (Err) 11 includes a phase correction circuit 18 composed of a series circuit of a capacitor C1 and a resistor R3 and a capacitor C2 connected in parallel to the series circuit of the capacitor C1 and the resistor R3. The phase correction circuit 18 is connected between one of input terminals of the error amplifier 11 and an output terminal thereof and constructs a negative feedback circuit functioning to prevent oscillation of the error amplifier 11 when the PWM drive gain is increased.
The comparator 13a of the PWM pulse generator circuit 13 compares a triangular wave signal generated by the triangular wave generator circuit 13b with the error voltage Ve to slice the tooth wave by the error voltage to thereby generate a PWM pulse signal, which is supplied to the driver 14. The driver 14 ON/OFF controls the transistor Q according to a width of the PWM pulse and generates a reduced voltage at the output terminal 16 when the DCxe2x80x94DC converter 10 is of a drop type. When the DCxe2x80x94DC converter is of a boost type, a boosted voltage, which is generated by a fly-back pulse, appears at the output terminal 16.
Incidentally, the Schottky diode D is a flywheel diode for returning a current, which flows out from the solenoid L when the transistor Q is turned OFF, to the solenoid L.
As described, the transistor Q of the DCxe2x80x94DC converter 10 is ON/OFF controlled such that the voltage Vs divided by the resistive voltage divider circuit 17 becomes coincident with the reference voltage Vref to make the output voltage at the output terminal 16 coincident with the constant voltage Vo to thereby stabilize the output voltage.
The input side power source line +Vcc, whose voltage Vcc is equal to Vin, is usually connected to a battery as shown by a dotted line. In a portable, notebook type personal computer, etc., however, one of a battery power source and an AC power source is selected by means of a power source switching circuit and utilized as the input side power source.
In a PWM control circuit for performing such operation, a range of the input power source voltage Vin is limited and its dynamic range is relatively narrow. The reason for this is that, in order to stabilize the output voltage when the input power source voltage is lowered, the PM control is performed such that a ratio of the ON period of an output transistor of the driver 14 to the OFF period thereof is set to, for example, 80% or more, so that a variation range of the pulse width with respect to a variation of the error voltage Ve is restricted and saturated. Therefore, it becomes difficult to satisfactorily perform a control operation with respect to a variation of the output voltage when the power source voltage is lowered. On the other hand, in the portable, notebook type personal computer, etc., in which one of the battery power source and the AC power source is selectively utilized as the input side power source by the power source switching circuit, the input power source voltage when the AC power source is used is usually higher than that of the battery. Therefore, when the AC power source is used, the variation of the power source voltage is larger than that when the battery is used, so that it becomes necessary to regulate the power source voltage in a wider range. Further, when the power source voltage is increased, the PWM control is performed in a level of the triangular wave in the vicinity of a summit thereof, contrarily to the case mentioned above. Therefore, it becomes difficult to perform a satisfactory control with respect to a variation of the output voltage when the power source voltage is lowered, similarly to the case mentioned above.
In order to obtain a stable output voltage under the circumstances, it is preferable that the PWM control is performed for the input power source voltage Vin in a level in which duty cycle is in a range from 30% to 70%.
As a control system capable of obtaining some dynamic range with respect to the input power source voltage Vin, there is the current mode PWM control system in which the PWM control is performed by current comparison. However, since, in such system, a detection resistor is inserted in series with a switching transistor Q, power loss is increased and a current comparator circuit, etc., are required additionally, so that there are problems of high cost and low power conversion efficiency.
On the other hand, in the conventional portable electronic device such as a notebook type personal computer, the output power source voltage is lowered in a standby state or a sleep mode thereof. On the contrary, in a case where luminance of a liquid crystal display thereof is set high, it is necessary to generate high output voltage. In such portable electronic device, it is necessary to give a wider dynamic range for the input power source voltage. However, the conventional DCxe2x80x94DC converter mentioned previously can not sufficiently deal with such requirements.
An object of the present invention is to provide a DCxe2x80x94DC converter, which is capable of making dynamic range of an input voltage wide and can stabilize an output voltage thereof even when a power source voltage is varied.
In order to achieve the above object, a DCxe2x80x94DC converter according to the present invention, which receives a predetermined power source voltage and controls an output voltage thereof such that the output voltage becomes equal to an aimed output voltage by switching an output transistor with a pulse having a predetermined width when the output voltage is lower than the aimed output voltage and stopping the switching operation of the output transistor when the output voltage exceeds the aimed voltage, is featured by comprising a variable duty cycle pulse generator circuit for generating a pulse having duty cycle, which is determined by a ratio of the power source voltage to the aimed output voltage, a switch circuit for outputting the pulse as a switching pulse of the output transistor when the output transistor is turned ON by the pulse and a control circuit for controlling the switch circuit to turn ON when the output voltage is lower than the aimed output voltage.
It is usual that, in the drop type DCxe2x80x94DC converter, which utilizes a switching regulator employing the PWM control system, the switching operation is controlled by a pulse having duty cycle, which is determined by a ratio of (input power source voltage)/(aimed output voltage to be stabilized). Therefore, in the present invention, the pulse having duty cycle corresponding to the ratio of the input power source voltage to the aimed output voltage is generated and the output transistor is switched by this pulse. In this case, the duty cycle is set to a value, which is in the vicinity of a center value thereof, that is, about 50%, in a voltage stabilizing control state when the input power source voltage Vin is a predetermined regular voltage. The output voltage is stabilized by performing the switching control of the output transistor with using the pulse having this duty cycle when the output voltage is lower than the aimed output voltage and stopping the switching operation of the transistor when the output voltage exceeds the aimed output voltage, without performing the PWM control.
In the present invention, the PWM control is performed by the variable duty cycle pulse generator circuit to deal with a variation of the input power source voltage Vin. That is, the PWM control is not used for the stabilizing control of the output voltage. When the input power source voltage is risen or lowered from the predetermined regular voltage, the PWM control is performed by the variable duty cycle pulse generator circuit correspondingly to the variation of the input power source voltage. That is, the PWM control is performed by reducing the duty cycle of the pulse when the input power source voltage is risen from the predetermined voltage or increasing the duty cycle when the input power source voltage is lowered. Therefore, the control range of the duty cycle can be changed from the center value thereof, which is 50%, so that it is possible to make the dynamic range of the DCxe2x80x94DC converter larger.
According to such control operation, even if an input power source voltage is supplied externally with which the duty cycle of the pulse is reduced from 50% to, for example, 25% or increased from 50% to 75%, it is possible to stabilize the output voltage by performing or stopping the switching control with using the duty cycle as the center value. Therefore, when the input power source is switched from the battery to the AC power source in, for example, a notebook type personal computer, it is possible to satisfactorily perform the output voltage stabilizing control.
Since, in the present invention, the pulse having duty cycle corresponding to a difference between the input power source voltage and the aimed output voltage is generated, it is possible to generate a pulse having duty cycle corresponding to the input voltage in the wide range and drive the switching transistor thereby. Further, since the duty cycle of the pulse is changed correspondingly to the input power source voltage, it is possible to control the input power source voltage with high traceability to a reduction thereof without using the PWM control and the variation range of duty cycle can be restricted to a small value.
As a result, it becomes possible to easily realize a DCxe2x80x94DC converter in which a wide dynamic range of the input voltage can be obtained and the output voltage thereof can be stabilized even when the power source voltage is varied.